1. Field of the Invention
The present invention relates to an improvement in a method for the production of polycrystalline silicon thin films. More particularly, the invention relates to a method for the production of polycrystalline silicon thin films comprising heat treatment of an as-deposited amorphous silicon thin film at relatively low temperatures under specific pressure, and to a polycrystalline silicon thin film having increased grain size prepared by the method.
2. Description of the Prior Art
Polycrystalline silicon thin films have received enormous commercial interest for their recent various applications in the fabrication of thin film transistors (TFT) for liquid crystal display (LCD), TFT for static random access memory (SRAM), solar cell, silicon on insulator (SOI), and the like. For such applications, the polycrystalline silicon thin films should have a grain size as large as possible in order to ensure high field effect mobility.
Several processes for preparing polycrystalline silicon thin films have been known. For example, Japanese Laid-Open Patent Publication No. 256,333/1991 discloses a process for preparing a semiconductor device comprising heat treatment of an amorphous silicon film coated on a single crystal semiconductor substrate at below 500.degree. C. to make the amorphous silicon film compact, and single crystallization of the compact amorphous silicon film by heat treatment at below 1,200.degree. C. in an inert gas atmosphere or a nitrogen gas atmosphere. The disclosed process comprises heat treatment at a high temperature of below 1,200.degree. C., resulting in exclusion of the possibility of using substrates having low melting temperatures. The resulting single crystallized silicon films have been described as having a grain size of about only 5 .mu.m.
Japanese Laid-Open Patent Publication No. 144,122/1992 discloses a process for preparing a polycrystalline silicon thin film by heat treatment at 600.degree. C. of a certain amorphous silicon film having a specific refractive index and extinction coefficient. The selected amorphous silicon films have been deposited by low pressure chemical vapor deposition (LPCVD) using expensive Si.sub.2 H.sub.6 rather than inexpensive SiH.sub.4 as a source gas. The resulting polycrystalline silicon thin films are found to have a grain size of 2.0-3.0 .mu.m.
It has been reported that recrystallization of an amorphous silicon film which has been deposited by LPCVD of silane at 640.degree. C. onto a thermally oxidized silicon wafer and has been amorphized by implantation of silicon ions, provides a polycrystalline silicon film having a grain size of 2.5 .mu.m at an anneal temperature of 580.degree. C. and 1.5 .mu.m at an anneal temperature of 640.degree. C. See, R. B. Iverson et at., J. Appl. Phys., Vol. 62, p.1675 (1987).
It has also been reported that the grain size of the polycrystalline silicon thin films recrystallized from amorphous silicon films which have been deposited by plasma chemical vapor deposition (PCVD) using a hydrogen-diluted SiH.sub.4 gas can be increased up to 4.0 .mu.m by decreasing the substrate temperature and nucleation rate. The amorphous silicon films are annealed at 600.degree. C. in a N.sub.2 atmosphere. See, Nakazawa et at., J. Appl. Phys., Vol.68(3), p.1029 (1990).
Nakazawa further reported in J. Appl. Phys., Vol. 69(3), p.1703-1706(1991) that the grain size of the polycrystalline silicon films recrystallized from Si.sub.2 H.sub.6 gas is larger than that from SiH.sub.4 gas. It has been reported that recrystallization of amorphous Si films deposited on a quartz substrate by LPCVD from Si.sub.2 H.sub.6 gas by annealing at 600.degree. C. in a N.sub.2 atmosphere gives poly-Si films having a grain size of 5 .mu.m.
It has been further known that the grain size of the polycrystalline silicon films crystallized from the amorphous silicon deposited by thermal CVD of disilane depends upon the deposition conditions; particularly it increases as the deposition rate increases. The deposition has been performed onto silicon wafers by LPCVD using a mixture of 5% disilane in helium as a source gas. Then, the as-deposited amorphous silicon films are subject to crystallization by annealing at 550.degree.-650.degree. C. See, Voutsas et at., J. Electrochem. Soc., Vol. 140, No.3, pp 871-877(1993).
However, no processes have yet been proposed which can provide polycrystalline silicon thin films having a grain size of above 5 .mu.m. Although disilane is more expensive than silane, the former is preferably used as a source gas. In addition, prior art processes have suffered from the drawback that heat treatment or annealing of amorphous silicon films should be performed at relatively high temperatures of above 600.degree. C., which excludes the possibility of use of low melting temperature materials, e.g., glass or ceramic plate, as a substrate. No research has been made on the effect of the pressure used during the heat treatment or during annealing of as-deposited amorphous silicon films on the grain size of the resulted polycrystalline thin films.
Therefore, in order to overcome the disadvantages encountered in the prior art techniques, we, the inventors of the present invention, conducted an intensive study on the production of polycrystalline silicon thin films, particularly on the effect of temperatures and pressures used in heat treatment for crystallization of deposited amorphous silicon films on the grain size. As a result, we have now achieved the present invention on the basis of the unexpected discovery that the annealing or heat treatment pressure of as-deposited amorphous silicon films influences mainly the grain size of the resulting polycrystalline silicon thin films.